Radio power amplifiers are used in telecommunications, for example, both for transmission and for reception. To operate a power amplifier efficiently, with maximum power gain, it is desirable to run it with bias voltage and input signal amplitude conditions such that it functions close to saturation, with the consequence that the linearity of its transfer function is degraded or distorted relative to an otherwise similar amplifier running further from saturation.
Radio power amplifiers that are made using monolithic integrated circuit (‘MIC’) technology are conveniently linearised by a pre-distorter using a scaled device of the same MIC technology as the power amplifier. Such a pre-distorter presents to the input signal a non-linear transfer function that tends to compensate the non-linearities in the operation of the power amplifier.
Many ways of building pre-distorters have already been described and many of them use a “linear/non-linear path” topology. One linear/non-linear path topology is described in U.S. Pat. No. 4,992,754 (Blauvelt & Loboda, assigned to Ortel corp., USA), and comprises a first path providing a copy of the input signal (linear path), and a second path in which some distortion is introduced through a non-linear device (non-linear path). The difference between the signals from the two paths contains a distortion that is arranged to be in opposition with the distortion of the power amplifier to linearise, in order to cancel out the distortion introduced by the power amplifier.
One limitation of the system described in that patent specification is that some delay must be introduced in the linear path to compensate for the delay in the non-linear path, and the relationship between the two paths in phase and amplitude is not easily maintained with frequency and from part to part. Another limitation is in the couplers, which may be well suited for hybrid technology but which are not easy to integrate, especially on semiconductor material such as Silicon where transmission lines are lossy.
U.S. Pat. No. 5,576,660 (Pouysegur & Nicolas assigned to Alcatel Espace, France) describes a pre-distorter using a linear/non-linear path topology with similar devices in the two paths. This helps a lot for frequency response because the electrical lengths (that is to say the phase difference between the input and output) of the two paths are similar and the characteristics of certain components of the pre-distorter vary in a similar manner with frequency and temperature. However, the pre-distorter comprises a 90° input divider and a 90° output coupler in hybrid form; the circuits of the two channels (or paths) are interconnected by microstrip circuits or by hybrid techniques. From the user point of view, numerous tuning operations are necessary in order to optimise the linearity of this system. Phase and gain need to be adjusted in each path to adjust the shape of the gain and phase pre-distortion curves. The bias voltages of all the active devices used in both paths need to be adjusted, and also the total gain of the pre-distorter to adjust its level characteristic to that of the power amplifier (the level at which the amplifier will start to introduce distortion).